Identification and distribution of type VI collagen in tendon fibrocartilages

Structural and compositional variations are marked in tendon fibrocartilages, which appear at the insertion to bone and in areas subjected to compressive plus frictional loading regime. We are interested in characterizing the extracellular matrix adaptations in these areas, in an attempt to understand cellular responses to biomechanics. In this work, we have applied immunocytochemistry and an ATP treatment for the ultrastructural identification of type VI collagen, to tendons subjected to compressive forces in different species. Immunocytochemistry, after testicular hyaluronidase or pepsin digestion, revealed the presence of type VI collagen in tensional and compressive areas of the plantaris longus tendon of the bullfrog, in the deep flexor tendon of dogs and rabbits, in the calcanear tendon and the suprapatela of rats and in the gastrocnemius tendon of chickens. In each tendon, the tensional region showed a weak reaction, restricted to the collagen fiber surface. However, the compression region was especially rich in type VI collagen, which accumulates in the interfibrillar spaces and is concentrated around the fibrochondrocytes. Intense reaction was also found in the paratenon. The ATP treatment not only allowed for the detection of the typical ladder-like aggregates of type VI collagen in the same areas identified by immunocytochemistry, but also demonstrated that type VI collagen forms a microfibrillar network around the fibrochondrocytes. Besides the function of organizing groups of collagen fibrils, type VI collagen seems to assemble the pericellular matrix in tendon fibrocartilages, perhaps through physical interactions with the large proteoglycans that concentrate in the same area.